RFC 6437

IPv6 Flow Label Specification

Internet Engineering Task Force (IETF) S. Amante
Request for Comments: 6437 Level 3
Obsoletes: 3697 B. Carpenter
Updates: 2205, 2460 Univ. of Auckland
Category: Standards Track S. Jiang
ISSN: 2070-1721 Huawei
J. Rajahalme
Nokia Siemens Networks
November 2011 IPv6 Flow Label Specification
Abstract
This document specifies the IPv6 Flow Label field and the minimum
requirements for IPv6 nodes labeling flows, IPv6 nodes forwarding
labeled packets, and flow state establishment methods. Even when
mentioned as examples of possible uses of the flow labeling, more
detailed requirements for specific use cases are out of the scope for
this document.
The usage of the Flow Label field enables efficient IPv6 flow
classification based only on IPv6 main header fields in fixed
positions.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6437.

1. Introduction
From the viewpoint of the network layer, a flow is a sequence of
packets sent from a particular source to a particular unicast,
anycast, or multicast destination that a node desires to label as a
flow. From an upper-layer viewpoint, a flow could consist of all
packets in one direction of a specific transport connection or media
stream. However, a flow is not necessarily 1:1 mapped to a transport
connection.
Traditionally, flow classifiers have been based on the 5-tuple of the
source address, destination address, source port, destination port,
and the transport protocol type. However, some of these fields may
be unavailable due to either fragmentation or encryption, or locating
them past a chain of IPv6 extension headers may be inefficient.
Additionally, if classifiers depend only on IP-layer headers, later
introduction of alternative transport-layer protocols will be easier.
The usage of the 3-tuple of the Flow Label, Source Address, and
Destination Address fields enables efficient IPv6 flow
classification, where only IPv6 main header fields in fixed positions
are used.
The flow label could be used in both stateless and stateful
scenarios. A stateless scenario is one where any node that processes
the flow label in any way does not need to store any information
about a flow before or after a packet has been processed. A stateful
scenario is one where a node that processes the flow label value
needs to store information about the flow, including the flow label
value. A stateful scenario might also require a signaling mechanism
to inform downstream nodes that the flow label is being used in a
certain way and to establish flow state in the network. For example,
RSVP [RFC2205] and General Internet Signaling Transport (GIST)
[RFC5971] can signal flow label values.
The flow label can be used most simply in stateless scenarios. This
specification concentrates on the stateless model and how it can be
used as a default mechanism. Details of stateful models, signaling,
specific flow state establishment methods, and their related service
models are out of scope for this specification. The basic
requirement for stateful models is set forth in Section 4.
The minimum level of IPv6 flow support consists of labeling the
flows. A specific goal is to enable and encourage the use of the
flow label for various forms of stateless load distribution,
especially across Equal Cost Multi-Path (ECMP) and/or Link
Aggregation Group (LAG) paths. ECMP and LAG are methods to bond
together multiple physical links used to procure the required

capacity necessary to carry an offered load greater than the
bandwidth of an individual physical link. Further details are in a
separate document [RFC6438]. IPv6 source nodes SHOULD be able to
label known flows (e.g., TCP connections and application streams),
even if the node itself does not require any flow-specific treatment.
Node requirements for stateless flow labeling are given in Section 3.
This document replaces [RFC3697] and Section 6 and Appendix A of
[RFC2460]. A rationale for the changes made is documented in
[RFC6436]. The present document also includes a correction to
[RFC2205] concerning the flow label.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
2. IPv6 Flow Label Specification
The 20-bit Flow Label field in the IPv6 header [RFC2460] is used by a
node to label packets of a flow. A Flow Label of zero is used to
indicate packets that have not been labeled. Packet classifiers can
use the triplet of Flow Label, Source Address, and Destination
Address fields to identify the flow to which a particular packet
belongs. Packets are processed in a flow-specific manner by nodes
that are able to do so in a stateless manner or that have been set up
with flow-specific state. The nature of the specific treatment and
the methods for flow state establishment are out of scope for this
specification.
Flow label values should be chosen such that their bits exhibit a
high degree of variability, making them suitable for use as part of
the input to a hash function used in a load distribution scheme. At
the same time, third parties should be unlikely to be able to guess
the next value that a source of flow labels will choose.
In statistics, a discrete uniform distribution is defined as a
probability distribution in which each value in a given range of
equally spaced values (such as a sequence of integers) is equally
likely to be chosen as the next value. The values in such a
distribution exhibit both variability and unguessability. Thus, as
specified in Section 3, an approximation to a discrete uniform
distribution is preferable as the source of flow label values.
Intentionally, there are no precise mathematical requirements placed
on the distribution or the method used to achieve such a
distribution.

Once set to a non-zero value, the Flow Label is expected to be
delivered unchanged to the destination node(s). A forwarding node
MUST either leave a non-zero flow label value unchanged or change it
only for compelling operational security reasons as described in
Section 6.1.
There is no way to verify whether a flow label has been modified en
route or whether it belongs to a uniform distribution. Therefore, no
Internet-wide mechanism can depend mathematically on unmodified and
uniformly distributed flow labels; they have a "best effort" quality.
Implementers should be aware that the flow label is an unprotected
field that could have been accidentally or intentionally changed en
route (see Section 6). This leads to the following formal rule:
o Forwarding nodes such as routers and load distributors MUST NOT
depend only on Flow Label values being uniformly distributed. In
any usage such as a hash key for load distribution, the Flow Label
bits MUST be combined at least with bits from other sources within
the packet, so as to produce a constant hash value for each flow
and a suitable distribution of hash values across flows.
Typically, the other fields used will be some or all components of
the usual 5-tuple. In this way, load distribution will still
occur even if the Flow Label values are poorly distributed.
Although uniformly distributed flow label values are recommended
below, and will always be helpful for load distribution, it is unsafe
to assume their presence in the general case, and the use case needs
to work even if the flow label value is zero.
As a general practice, packet flows should not be reordered, and the
use of the Flow Label field does not affect this. In particular, a
Flow label value of zero does not imply that reordering is
acceptable.
3. Flow Labeling Requirements in the Stateless Scenario
This section defines the minimum requirements for methods of setting
the flow label value within the stateless scenario of flow label
usage.
To enable Flow-Label-based classification, source nodes SHOULD assign
each unrelated transport connection and application data stream to a
new flow. A typical definition of a flow for this purpose is any set
of packets carrying the same 5-tuple {dest addr, source addr,
protocol, dest port, source port}. It should be noted that a source
node always has convenient and efficient access to this 5-tuple,
which is not always the case for nodes that subsequently forward the
packet.

It is desirable that flow label values should be uniformly
distributed to assist load distribution. It is therefore RECOMMENDED
that source hosts support the flow label by setting the flow label
field for all packets of a given flow to the same value chosen from
an approximation to a discrete uniform distribution. Both stateful
and stateless methods of assigning a value could be used, but it is
outside the scope of this specification to mandate an algorithm. The
algorithm SHOULD ensure that the resulting flow label values are
unique with high probability. However, if two simultaneous flows are
assigned the same flow label value by chance and have the same source
and destination addresses, it simply means that they will receive the
same flow label treatment throughout the network. As long as this is
a low-probability event, it will not significantly affect load
distribution.
A possible stateless algorithm is to use a suitable 20-bit hash of
values from the IP packet's 5-tuple. A simple example hash function
is described in Appendix A.
An alternative approach is to use a pseudo-random number generator to
assign a flow label value for a given transport session; such a
method will require minimal local state to be kept at the source node
by recording the flow label associated with each transport socket.
Viewed externally, either of these approaches will produce values
that appear to be uniformly distributed and pseudo-random.
An implementation in which flow labels are assigned sequentially is
NOT RECOMMENDED, as it would then be simple for on-path observers to
guess the next value.
A source node that does not otherwise set the flow label MUST set its
value to zero.
A node that forwards a flow whose flow label value in arriving
packets is zero MAY change the flow label value. In that case, it is
RECOMMENDED that the forwarding node sets the flow label field for a
flow to a uniformly distributed value as just described for source
nodes.
o The same considerations apply as to source hosts setting the flow
label; in particular, the preferred case is that a flow is defined
by the 5-tuple. However, there are cases in which the complete
5-tuple for all packets is not readily available to a forwarding
node, in particular for fragmented packets. In such cases, a flow
can be defined by fewer IPv6 header fields, typically using only
the 2-tuple {dest addr, source addr}. There are alternative
approaches that implementers could choose, such as:

* A forwarding node might use the 5-tuple to define a flow
whenever possible but use the 2-tuple when the complete 5-tuple
is not available. In this case, unfragmented and fragmented
packets belonging to the same transport session would receive
different flow label values, altering the effect of subsequent
load distribution based on the flow label.
* A forwarding node might use the 2-tuple to define a flow in all
cases. In this case, subsequent load distribution would be
based only on IP addresses.
o The option to set the flow label in a forwarding node, if
implemented, would presumably be of value in first-hop or ingress
routers. It might place a considerable per-packet processing load
on them, even if they adopted a stateless method of flow
identification and label assignment. However, it will not
interfere with host-to-router load sharing [RFC4311]. It needs to
be under the control of network managers, to avoid unwanted
processing load and any other undesirable effects. For this
reason, it MUST be a configurable option, disabled by default.
The preceding rules taken together allow a given network to include
routers that set flow labels on behalf of hosts that do not do so.
The complications described explain why the principal recommendation
is that the source hosts should set the label.
4. Flow State Establishment Requirements
A node that sets the flow label MAY also take part in a flow state
establishment method that results in assigning specific treatments to
specific flows, possibly including signaling. Any such method MUST
NOT disturb nodes taking part in the stateless scenario just
described. Thus, any node that sets flow label values according to a
stateful scheme MUST choose labels that conform to Section 3 of this
specification. Further details are not discussed in this document.
5. Essential Correction to RFC 2205
[RFC2460] reduced the size of the flow label field from 24 to 20
bits. The references to a 24-bit flow label field in Section A.9 of
[RFC2205] are updated accordingly.
6. Security Considerations
This section considers security issues raised by the use of the Flow
Label, including the potential for denial-of-service attacks and the
related potential for theft of service by unauthorized traffic
(Section 6.2). Section 6.3 addresses the use of the Flow Label in

the presence of IPsec, including its interaction with IPsec tunnel
mode and other tunneling protocols. We also note that inspection of
unencrypted Flow Labels may allow some forms of traffic analysis by
revealing some structure of the underlying communications. Even if
the flow label was encrypted, its presence as a constant value in a
fixed position might assist traffic analysis and cryptoanalysis.
The flow label is not protected in any way, even if IPsec
authentication [RFC4302] is in use, so it can be forged by an on-path
attacker. Implementers are advised that any en-route change to the
flow label value is undetectable. On the other hand, a uniformly
distributed pseudo-random flow label cannot be readily guessed by an
attacker; see [LABEL-SEC] for further discussion. If a hash
algorithm is used, as suggested in Section 3, it SHOULD include a
step that makes the flow label value significantly difficult to
predict [RFC4086], even with knowledge of the algorithm being used.
6.1. Covert Channel Risk
The flow label could be used as a covert data channel, since
apparently pseudo-random flow label values could, in fact, consist of
covert data [NSA]. This could, for example, be achieved using a
series of otherwise innocuous UDP packets whose flow label values
constitute a covert message, or by co-opting a TCP session to carry a
covert message in the flow labels of successive packets. Both of
these could be recognized as suspicious -- the first because isolated
UDP packets would not normally be expected to have non-zero flow
labels, and the second because the flow label values in a given TCP
session should all be equal. However, other methods, such as co-
opting the flow labels of occasional packets, might be rather hard to
detect.
In situations where the covert channel risk is considered
significant, the only certain defense is for a firewall to rewrite
non-zero flow labels. This would be an exceptional violation of the
rule that the flow label, once set to a non-zero value, must not be
changed. To preserve load distribution capability, such a firewall
SHOULD rewrite labels by following the method described for a
forwarding node (see Section 3), as if the incoming label value were
zero, and MUST NOT set non-zero flow labels to zero. This behavior
is nevertheless undesirable, since (as discussed in Section 3) only
source nodes have straightforward access to the complete 5-tuple.
6.2. Theft and Denial of Service
Since the mapping of network traffic to flow-specific treatment is
triggered by the IP addresses and Flow Label value of the IPv6
header, an adversary may be able to obtain a class of service that

the network did not intend to provide by modifying the IPv6 header or
by injecting packets with false addresses and/or labels. A concrete
analysis of this threat is only possible for specific stateful
methods of signaling and using the flow label, which are out of scope
for this document. Clearly, a full analysis will be required when
any such method is specified, but in general, networks SHOULD NOT
make resource allocation decisions based on flow labels without some
external means of assurance.
A denial-of-service attack [RFC4732] becomes possible in the
stateless model when the modified or injected traffic depletes the
resources available to forward it and other traffic streams. If a
denial-of-service attack were undertaken against a given Flow Label
(or set of Flow Labels), then traffic containing an affected Flow
Label might well experience worse-than-best-effort network
performance.
Note that since the treatment of IP headers by nodes is typically
unverified, there is no guarantee that flow labels sent by a node are
set according to the recommendations in this document. A man-in-the-
middle or injected-traffic denial-of-service attack specifically
directed at flow label handling would involve setting unusual flow
labels. For example, an attacker could set all flow labels reaching
a given router to the same arbitrary non-zero value or could perform
rapid cycling of flow label values such that the packets of a given
flow will each have a different value. Either of these attacks would
cause a stateless load distribution algorithm to perform badly and
would cause a stateful classifier to behave incorrectly. For this
reason, stateless classifiers should not use the flow label alone to
control load distribution, and stateful classifiers should include
explicit methods to detect and ignore suspect flow label values.
Since flows are identified by the 3-tuple of the Flow Label and the
Source and Destination Address, the risk of denial of service
introduced by the Flow Label is closely related to the risk of denial
of service by address spoofing. An adversary who is in a position to
forge an address is also likely to be able to forge a label, and vice
versa.
There are two issues with different properties: spoofing of the Flow
Label only and spoofing of the whole 3-tuple, including Source and
Destination Address.
The former can be done inside a node that is using or transmitting
the correct source address. The ability to spoof a Flow Label
typically implies being in a position to also forge an address, but

in many cases, spoofing an address may not be interesting to the
spoofer, especially if the spoofer's goal is theft of service rather
than denial of service.
The latter can be done by a host that is not subject to ingress
filtering [RFC2827] or by an intermediate router. Due to its
properties, this is typically useful only for denial of service. In
the absence of ingress filtering, almost any third party could
instigate such an attack.
In the presence of ingress filtering, forging a non-zero Flow Label
on packets that originated with a zero label, or modifying or
clearing a label, could only occur if an intermediate system such as
a router was compromised, or through some other form of man-in-the-
middle attack.
6.3. IPsec and Tunneling Interactions
The IPsec protocol, as defined in [RFC4301], [RFC4302], and
[RFC4303], does not include the IPv6 header's Flow Label in any of
its cryptographic calculations (in the case of tunnel mode, it is the
outer IPv6 header's Flow Label that is not included). Hence,
modification of the Flow Label by a network node has no effect on
IPsec end-to-end security, because it cannot cause any IPsec
integrity check to fail. As a consequence, IPsec does not provide
any defense against an adversary's modification of the Flow Label
(i.e., a man-in-the-middle attack).
IPsec tunnel mode provides security for the encapsulated IP header's
Flow Label. A tunnel mode IPsec packet contains two IP headers: an
outer header supplied by the tunnel ingress node and an encapsulated
inner header supplied by the original source of the packet. When an
IPsec tunnel is passing through nodes performing flow classification,
the intermediate network nodes operate on the Flow Label in the outer
header. At the tunnel egress node, IPsec processing includes
removing the outer header and forwarding the packet (if required)
using the inner header. The IPsec protocol requires that the inner
header's Flow Label not be changed by this decapsulation processing
to ensure that modifications to the label cannot be used to launch
theft- or denial-of-service attacks across an IPsec tunnel endpoint.
This document makes no change to that requirement; indeed, it forbids
changes to the Flow Label.
When IPsec tunnel egress decapsulation processing includes a
sufficiently strong cryptographic integrity check of the encapsulated
packet (where sufficiency is determined by local security policy),
the tunnel egress node can safely assume that the Flow Label in the
inner header has the same value it had at the tunnel ingress node.

This analysis and its implications apply to any tunneling protocol
that performs integrity checks. Of course, any Flow Label set in an
encapsulating IPv6 header is subject to the risks described in the
previous section.
6.4. Security Filtering Interactions
The Flow Label does nothing to eliminate the need for packet
filtering based on headers past the IP header if such filtering is
deemed necessary for security reasons on nodes such as firewalls or
filtering routers.
7. Differences from RFC 3697
The main differences between this specification and its predecessor
[RFC3697] are as follows:
o This specification encourages non-zero flow label values to be
used and clearly defines how to set a non-zero value.
o This specification encourages a stateless model with uniformly
distributed flow label values.
o This specification does not specify any details of a stateful
model.
o This specification retains the rule that the flow label must not
be changed en route but allows routers to set the label on behalf
of hosts that do not do so.
o This specification discusses the covert channel risk and its
consequences for firewalls.
For further details, see [RFC6436].
8. Acknowledgements
Valuable comments and contributions were made by Jari Arkko, Ran
Atkinson, Fred Baker, Richard Barnes, Steve Blake, Tassos
Chatzithomaoglou, Remi Despres, Alan Ford, Fernando Gont, Brian
Haberman, Tony Hain, Joel Halpern, Qinwen Hu, Chris Morrow, Thomas
Narten, Mark Smith, Pascal Thubert, Iljitsch van Beijnum, and other
participants in the 6man working group.
Cristian Calude suggested the von Neumann algorithm in Appendix A.
David Malone and Donald Eastlake gave additional input about hash
algorithms.

Appendix A. Example 20-Bit Hash Function
As mentioned in Section 3, a stateless hash function may be used to
generate a flow label value from an IPv6 packet's 5-tuple. It is not
trivial to choose a suitable hash function, and it is expected that
extensive practical experience will be required to identify the best
choices. An example function, based on an algorithm by von Neumann
known to produce an approximately uniform distribution [vonNeumann],
follows. For each packet for which a flow label must be generated,
execute the following steps:
1. Split the destination and source addresses into two 64-bit values
each, thus transforming the 5-tuple into a 7-tuple.
2. Add the following five components together using unsigned 64-bit
arithmetic, discarding any carry bits: both parts of the source
address, both parts of the destination address, and the protocol
number.
3. Apply the von Neumann algorithm to the resulting string of 64
bits:
1. Starting at the least significant end, select two bits.
2. If the two bits are 00 or 11, discard them.
3. If the two bits are 01, output a 0 bit.
4. If the two bits are 10, output a 1 bit.
5. Repeat with the next two bits in the input 64-bit string.
6. Stop when 16 bits have been output (or when the 64-bit string
is exhausted).
4. Add the two port numbers to the resulting 16-bit number.
5. Shift the resulting value 4 bits left, and mask with 0xfffff.
6. In the highly unlikely event that the result is exactly zero, set
the flow label arbitrarily to the value 1.
Note that this simple example does not include a step to prevent
predictability, as recommended in Section 6.